Catalysts

ABSTRACT

Shaped particles, comprising a calcium aluminate cement and at least one oxide of a Group VIII metal M selected from nickel and cobalt, said particles containing 10 to 70% by weight of said Group VIII metal oxide (expressed as the divalent oxide, MO) and having a porosity in the range 25 to 50%, in which at least 30% of the pore volume is in the form of pores of size in the range 15 to 35 nm and less than 40% of the pore volume is in the form of pores of diameter greater than 35 nm. These particles are used to catalytically decompose oxidizing agents including hypohalites.

This is a division of application Ser. No. 07/519,690, filed May 7,1990, now U.S. Pat. No. 5,041,408.

This invention relates to catalysts and in particular to catalysts, orprecursors thereto, containing an inert support material and at leastone oxide of a metal of Group VIII of the Periodic Table and selectedfrom nickel and cobalt.

For some catalytic applications the Group VIII metal oxide is thecatalytically active species while for other catalytic applications theGroup VIII metal oxide is a catalyst precursor and the catalyticallyactive species is the product of reducing the Group VIII metal oxide tothe Group VIII metal or is the product of oxidising the Group VIII metaloxide in the precursor to a higher oxidation state. For examplecatalysts obtained by reduction of a precursor containing nickel and/orcobalt oxide are of use as hydrogenation catalysts, e.g. methanationcatalysts for the hydrogenation of carbon oxides to methane or catalystsfor the hydrogenation of aromatic compounds such as benzene tocyclohexane. Another use of supported nickel and/or cobalt oxides is ascatalysts for the decomposition of oxidising agents such as hypochloriteions in aqueous solutions, for example in the treatment of effluentscontaining such ions prior to discharge of into rivers, lakes, orestuaries.

Aqueous media containing oxidising agents often have a relatively highpH: in order to prevent the catalyst particles disintegrating during useas a result of such alkaline conditions, it has previously been proposedto employ Group VIII metal oxides in intimate mixture with a metal oxidesuch as magnesia or alumina as a coating on a non-porous, hard fired,support of e.g. alumina or magnesia (see U.S. Pat. No. 4,732,688 and EP276044) or to employ a resin binder, for example polyvinylidenefluoride, (see U.S. Pat. No. 4,400,304 and the reissue thereof U.S. Pat.No. Re. 32,392). Catalysts made using the aforesaid inert non-poroussupports have a relatively small Group VIII metal oxide content,typically less than 10% by weight, and usually less than 5%. While theinitial activity of such materials is good, the service life of suchcatalysts is in some cases inadequate, as a result of loss of activematerial through loss of adhesion of the coating to the support. Whileparticles made using a resin binder as aforesaid may have a somewhatgreater Group VIII metal oxide content, the binder tends to restrictaccess of the effluent being treated to the active catalyst and also thebinders necessary to withstand highly alkaline conditions are relativelyexpensive.

We have devised catalysts, and precursors thereto, in the form ofparticles containing substantial proportions of the Group VIII metaloxide and in which the active material is readily accessible to thereactants. This invention is based on the finding that, surprisingly,calcium aluminate cement can withstand highly alkaline conditions.

It has been proposed in GB 1278424 to produce a methanation catalystprecursor containing nickel oxide and calcium aluminate cement in theform of spherical agglomerates having a low bulk density (in the range0.4 to 0.656 g.cm⁻³), a high porosity (in the range 55 to 75%) with ahigh proportion of the pores (50-70% of the total pore volume) in theform of "macropores" of diameter greater than 35 nm. We have found thatmaterials of lower porosity and having a relatively low proportion ofpores in the form of such macropores, are particularly suitable for thedecomposition of oxidising agents in aqueous media.

Accordingly the present invention provides shaped particles suitable foruse as a catalyst, or precursor thereto, comprising a calcium aluminatecement and at least one oxide of a Group VIII metal M selected fromnickel and cobalt, said particles containing 10 to 70% by weight of saidGroup VIII metal oxide (expressed as the divalent oxide, MO) and havinga porosity in the range 25 to 50%, particularly 30 to 50%, in which atleast 30% of the pore volume is in the form of pores of size in therange 15 to 35 nm and less than 40% of the pore volume is in the form ofpores of diameter greater than 35 nm.

The shaped particles are preferably in the form of granules, extrudates,or pellets and preferably have an aspect ratio, by which we mean theratio of the weight average maximum geometric dimension, e.g. length, toweight average minimum geometric dimension, e.g. diameter, of less than3, particularly less than 2. Particles having a greater aspect ratio maybe liable to suffer from breakage during use. The shaped particlespreferably have a weight average maximum dimension in the range 2 to 8mm, particularly 3 to 8 mm. This ensures that the particles have arelatively high a geometric surface area per unit bed volume, so that abed of the particles has a relatively large external particle areaexposed to the reactants without the presence of an undue proportion offines which would lead to unacceptable pressure drop on passage ofreactants through a bed of the particles.

The particles of the invention have a porosity in the range 30 to 50%.By the term porosity we mean the ratio of the volume of the pores to thevolume of the particle. Porosity may be determined by measurement of themercury and helium densities of the particles: the porosity (as apercentage) is given by

    porosity=p.sub.Hg ×[1/p.sub.Hg -1/p.sub.He ]×100

where p_(Hg) and p_(He) are respectively the mercury and heliumdensities.

As the porosity of the particles increases they become weaker: aporosity in the range 25 to 50%, particularly 30 to 50%, allows thereactants to have ready access to the active material within theparticles but is not so large that the particles have inadequatestrength. Also, the particles of the invention have a particular poresize distribution. This may be determined by mercury intrusionporosimetry. In the particles of the invention, at least 30%, andpreferably 40 to 70%, of the pore volume is in the form of pores ofaverage diameter in the range 15-35 nm and less than 40% of the porevolume is in the form of pores of average diameter above 35 nm.Particles having such a pore size are of particular utility where theyare used for the decomposition of oxidising agents in aqueous media, asin that treatment, gases such as oxygen may be liberated and it isthought that if the pores were too small, such liberated gases wouldtend to fracture the particles, while if the pores are too large, theparticles may have only a short useful life as they become undulyweakened by the slow dissolution of the support material during use.While particles of low porosity, for example as produced by standardtabletting techniques, are stronger and less liable to such fracture byliberated gases, the low porosity restricts access of the reactants tothe active material and as a result the activity of the catalyst isimpaired.

Largely as a result of the porosity and pore size distribution, theparticles also have a relatively high BET surface area, above 10, and inparticular in the range 20-100, m².g⁻¹. As a result the active materialis present in a finely divided state. Such a BET surface area may beachieved by introducing the Group VIII metal oxide into the compositionby a precipitation route as described hereinafter.

As a result of their composition and porosity, the shaped particles ofthe invention have a bulk density in the range 0.8 to 1.5, preferably0.9 to 1.4, g.cm⁻³. The bulk density is indicative of the weight ofcatalyst in a bed of given volume.

During use of the particles as a catalyst for the decomposition ofoxidising agents, e.g. in effluents, the BET surface area, porosityand/or pore size distribution may change: thus the BET surface area,porosity, and the proportion of pores of size less than 35 nm mayincrease. The surface area, density, and porosity parameters of theshaped particles referred to herein refer to the parameters of theparticles in the "as made" state, i.e. before use for catalyticpurposes.

Shaped particles having the required porosity and pore volumecharacteristics may be made by a particular pelleting method asdescribed hereinafter: shaped particles made by conventional tablettingmachines have a considerably lower porosity and only a low percentage ofthe pore volume is in the form of pores of size above 15 nm, whileshaped particles made by the agglomeration method described in GB1278424 have a large proportion of pores of size above 35 nm.

The composition comprises at least one oxide of a Group VIII metalselected from nickel and cobalt. Preferably the Group VIII metal isnickel alone, or nickel in admixture with cobalt in an amount of up toone mole of cobalt per mole of nickel. The composition also comprises acalcium aluminate cement, by which term we mean hydraulic cementscontaining one or more calcium aluminate compounds of the formulanCaO.mAl₂ O₃ where n and m are integers. Examples of such calciumaluminate compounds incluse calcium mono-aluminate CaO.Al₂ O₃,tri-calcium aluminate 3CaO.Al₂ O₃, penta-calcium tri-aluminate 5CaO.3Al₂O₃, tri-calcium penta-aluminate 3CaO.5Al₂ O₃, and dodeca-calciumhepta-aluminate 12CaO.7Al₂ O₃.

Calcium aluminate cements are often contaminated with iron compounds.The presence of iron compounds is generally undesirable in compositionswhere the shaped particles are to be used, after a reduction step, ascatalysts for example for hydrogenation reactions, and so for suchapplications a calcium aluminate cement that has a low iron content isdesirable. However, for use in treating fluid, e.g. aqueous, mediacontaining oxidising agents, the presence of substantial amounts, e.g. 5to 20% by weight of iron oxide (expressed as Fe₂ O₃), in the calciumaluminate cement can be tolerated and indeed may in some cases bebeneficial as the iron oxide may act as a an activity promoter. Althoughiron oxides are amphoteric, because the iron oxide is bound into thecement, it does not leach out to any significant extent even underhighly alkaline conditions such as are likely to be encountered in thetreatment of aqueous medium containing hypochlorite ions. Where theshaped particles are to be used for the decomposition of oxidisingagents in aqueous media, it is preferred that they contain 0.2 to 10% byweight of iron oxide expressed as Fe₂ O₃. A particularly suitablecalcium aluminate cement is that known as "ciment fondu".

As mentioned hereinbefore, the Group VIII metal oxide is preferablyintroduced into the composition by precipitation. A preferred route isto precipitate Group VIII metal compounds, decomposable to oxides byheating, from an aqueous solution of e.g. nitrates by addition of aprecipitant such as an alkali metal carbonate solution. Afterprecipitation of the Group VIII metal compounds, the precipitate iswashed free of precipitant. The precipitate may be mixed with a finelydivided, preferably inert, diluent material, such as magnesia (which, asshown in the aforesaid EP 276044 may have a beneficial effect on theactivity of the catalyst) and/or a clay, e.g. kaolin. The amount of suchdiluent material employed is conveniently up to twice the weight of theGroup VIII metal compounds expressed as the divalent oxides. The mixtureis then dried, and calcined, e.g. to a temperature in the range200°-600° C., particularly 400°-550° C., to effect decomposition of theGroup VIII metal compounds to the oxide form. Minor amounts of otheringredients, such as co-promoters such as magnesium oxide may beincorporated, e.g. by co-precipitation with the Group VIII metalcompounds. The resultant composition is then mixed with the calciumaluminate cement, optionally together with a processing aid such as alittle water, a stearate of an alkaline earth metal, e.g. magnesium,and/or graphite, and formed into pellets. The proportion of cementemployed is generally 25 to 100% by weight based on the total weight ofthe Group VIII metal oxide, or oxides, and any diluent material, and issuch as to give a composition containing 10 to 70%, particularly lessthan 50%, and most preferably 20 to 40%, by weight of the Group VIIImetal oxide or oxides.

In order to obtain shaped particles of the requisite pore volumecharacteristics, the mixture is conveniently pelletised by means of apellet mill, for example of the type used for pelleting animalfeedstuffs, wherein the mixture to be pelleted is charged to a rotatingperforate cylinder through the perforations of which the mixture isforced by a bar or roller within the cylinder. The resulting extrudedmixture is cut from the surface of the rotating cylinder by a doctorknife positioned to give pellets of the desired length. It will beappreciated that other extrusion techniques may be employed to giveshaped particles of the desired characteristics.

After forming the composition into the desired shaped particles, thelatter are then preferably contacted with water, preferably as steam, toeffect hydration of the cement and to give the shaped particles adequatestrength.

Shaped particles formed by this method have a significantly lowerstrength, e.g. as measured by a crushing test, than pellets prepared bya conventional tabletting technique but it is found that, even so, thestrength is adequate for the applications envisaged and, indeed, thestrength generally increases where the catalyst is employed for thedecomposition of oxidising agents in aqueous media, presumably as aresult of continued hydration of the cement.

For use for the decomposition of oxidising agents, the catalyst bed iscontacted with a fluid medium, particularly aqueous, containing theoxidising agent to be treated. Examples of oxidising agents that may bedecomposed using the shaped particles of the invention includehypohalite ions, for example hypochlorite and hypobromite ions, andhydrogen peroxide. At least some of such oxidising agents are pollutantsin various industrial processes. In particular hypochlorite ions are asignificant industrial pollutant.

Conveniently a fixed bed of the catalyst particles is formed and themedium containing the oxidising agent, for example hypochlorite ions, ispassed through the bed. Generally the medium is in the form of anaqueous solution which has been filtered prior to contact with thecatalyst bed.

The treatment of aqueous media is conveniently effected under conditionssuch that the pH of the medium is above 7, preferably above 8; it is aparticularly beneficial aspect of the invention that the particles donot physically disintegrate even at pH levels in the range 10 to 14. Theprocess can be performed at any convenient temperature, suitably in therange 5°-100° C., more suitably in the range 20°-80° C.

When the shaped particles are contacted with the oxidising agent in anaqueous medium, some or all of the oxides of the particles may becomehydrated. In addition the Group VIII metal oxides are oxidised to highervalency states. For example nickel oxide can be notionally considered tobe initially present in the particles as NiO. Authorities vary as toprecisely what higher oxides of nickel are formed but it may be regardedthat the higher oxides Ni₃ O₄, Ni₂ O₃ and NiO₂ are formed on contactwith the oxidising agent. Such higher oxides are active in the processof decomposition of the oxidising agent. In the particles of the presentinvention, the Group VIII metal oxides may be as initially formed or intheir higher oxidation states, as formed in use. In use the oxides mayalso be present as hydrates. It should be noted, however, that theproportions specified herein of the Group VIII metal oxide in theparticles are expressed on the basis of anhydrous oxides with the GroupVIII oxides in the divalent state, i.e. NiO and/or CoO.

In addition to use for the decomposition of oxidising agents asdescribed above, the shaped particles of the invention are also of useas precursors to hydrogenation catalysts, and may be converted to thecatalytically active form by reduction, e.g. with a stream of ahydrogen-containing gas at an elevated temperature. Such reduction maybe effected after charging the particles to a vessel in which thehydrogenation is to be effected. Alternatively, the reduction may beeffected as a separate step prior to charging the particles to thehydrogenation reactor and, if desired, the reduced particles may bepassivated by contact with a gas stream containing a small amount ofoxygen, or with carbon dioxide followed by a gas stream containing asmall amount of oxygen, until no further reaction occurs when theparticles may then be handled in air at ambient temperature.

The invention is illustrated by the following examples in which allparts and percentages are by weight.

EXAMPLE 1

A slurry containing precipitated basic nickel carbonate, and a mixtureof finely divided magnesia and kaolin as diluent materials, wasfiltered, washed, dried, and then calcined at 400°-450° C. The slurrycontained about 17 parts of magnesia and 104 parts of kaolin per 100parts of nickel, so that the calcined material contained about 13.3parts of magnesia and about 82 parts of kaolin per 100 parts of nickeloxide.

100 parts of the calcined material were then mixed with about 2 parts ofgraphite and 50 parts of a calcium aluminate cement having an aluminiumto calcium atomic ratio of about 1.4 and having an iron content,expressed as Fe₂ O₃, of about 15%, to give a dry feed mixture.

The dry feed mixture was then mixed with water (25 parts per 100 partsof the cement-containing mixture), formed into extruded pellets ofdiameter of about 3 mm and lengths in the range of about 3 to 5 mm usinga pellet mill as described hereinbefore, and then dried to giveextrudates A.

For purposes of comparison a similar dry feed mixture was formed intotablets of diameter and length about 3 mm using a conventionaltabletting machine, and then the resulting tablets were soaked in waterat room temperature, and then dried to give tablets B.

Further for purposes of comparison, a granulated product was madefollowing the procedure of GB 1278424. Thus a dry powder feed mixture ofthe calcined material and calcium aluminate cement was made up as in theproduction of extrudates A except that the graphite was omitted. Thismixture was granulated, with water addition, using a disc granulator of20 cm diameter. After initial scouting experiments to determine suitableconditions, a tilt angle of 60°, a speed of 30 rpm, and a feed of thedry powder, via a vibrating feeder, at an overall rate of about 30g/min, alternating with spraying of water on to the dish in bursts ofabout 20-40 seconds, were found to give granules, wherein about 50% byweight had a size within the range 2-4 mm, and were otherwise similar tothose described in GB 1278424. It was found that, in order to obtaingranules that could be handled easily without breakage, the amount ofwater required, about 75 ml per 100 g of dry feed, was rather more thanindicated in the aforesaid GB 1278424. Using those conditions, granuleswere produced and the product sieved to select those granules, whichwere designated granules C, of size range 2-4 mm.

The physical properties of the extrudates A, tablets B, and granules C,are set out in Table 1.

120 ml of the extrudates A were charged to a reactor of internaldiameter 2.5 cm to form a catalyst bed therein. Another reactor of thesame size was charged with 120 ml of tablets B. A feed of an aqueoussolution containing sodium hypochlorite and sodium hydroxide and havinga pH of about 12.5 was preheated to about 60° C. was fed to the reactorsin parallel so that the hypochlorite solution flowed up through thecatalyst bed. The concentration of hypochlorite in the feed, the spacevelocity (SV) through each bed, and the exit sodium hypochloriteconcentration were monitored. The results are shown in Table 2. FromTable 2 it is seen that, after an initial period, the activity of theextrudates is significantly greater than that of the tablets B.

When granules C were tested in similar fashion, they showed highactivity, as good as or better than extrudates A, but the outlet linewas always dark with suspended solids--analysis revealed that theeffluent from the catalyst bed typically contained 78 ppm of combinednickel, indicating that the catalyst was rapidly being leached away. (Incontrast, the effluent from the bed of extrudates A typically containedonly 1.8 ppm of combined nickel). The high nickel content of theeffluent from the bed of granules C could well explain their apparenthigh activity as the leached nickel would continue to act as catalystfor hypochlorite decomposition after the effluent has left the catalystbed: as a consequence the effective space velocity is very low. After atotal of about 1600 hours, granules C had disintegrated and leached tosuch an extent that only about 20% of the original volume of catalystcharged to the reactor remained therein.

                  TABLE 1                                                         ______________________________________                                                      Extrud. A                                                                             Tablets B                                                                              Granules C                                     ______________________________________                                        Analysis* (%)                                                                 (NiO            30.6           35.2                                           (Al.sub.2 O.sub.3                                                                             30.5           25.3                                           (CaO            12.0           13.3                                           (Fe.sub.2 O.sub.3                                                                             5.5            6.0                                            (MgO            3.7            4.1                                            (SiO.sub.2      16.0           14.6                                           (Other**        1.7            1.5                                            BET surface area (m.sup.2 · g.sup.-1)                                                31.7      37.0     91.0                                       Helium density (g · cm.sup.-3)                                                       2.84      2.87     3.04                                       Mercury density (g · cm.sup.-3)                                                      1.81      2.23     0.96                                       Bulk density (g · cm.sup.-3)                                                         0.95      1.08     0.61                                       Porosity (%)    36        22       68                                         Pores >35 nm (%)                                                                              29        25       59                                         Pores 15-35 nm (%)                                                                            44        25       33                                         Crush strength (kg).sup.+                                                                     5.3       15.8     <1                                         ______________________________________                                         *After ignition at 900° C.                                             **Minor components such as TiO.sub.2 and alkali metal oxides resulting        from contaminants in the starting materials or introduced during the          formation of the shaped particles.                                            .sup.+ Mean load required to crush particles with the load applied            perpendicular to the longitudinal axis of the shaped particles. The quote     figure is the value obtained on the particles as made: because further        hydration of the cement may occur, the strength may increase during use. 

                  TABLE 2                                                         ______________________________________                                        space      NaOCl concn. (g/100 ml)                                                                         NaOCl                                            time  velocity        outlet       removed (%)                                (hr)  (hr.sup.-1)                                                                            feed   Extruds. A                                                                            Tablets B                                                                            A     B                                  ______________________________________                                         72   1.30     7.19   0.086          98.8                                      96   2.32     6.85           0.11         98.4                                936  0.88     8.38           0.13         98.4                               1416  1.33     5.89   0.112          98.1                                     4100  1.00     6.14   0.006          99.9                                     4344  1.10     6.14           0.14         97.7                               5112  1.39     5.57           0.16         97.1                               6310  1.04     6.12   0.008          99.9                                     7128  1.20     7.59   0.010          99.9                                     7368  1.51     7.59           0.28         96.3                               11664 0.80     8.04   0.030   0.28   99.6  96.5                               ______________________________________                                    

In the following Examples 2-7, the procedure of Example 1 used to makeextrudates A was repeated with minor modifications, including in eachcase the use of a pellet mill producing extrudates of diameter about 1.6mm and lengths in the range of about 3 to 5 mm.

EXAMPLE 2

In this example the slurry of the precipitated basic nickel carbonateand the mixture of the finely divided diluent materials contained ahigher proportion of the diluent materials, thereby giving a product oflower nickel oxide content. The product was designated extrudates D.

EXAMPLE 3-5

In these examples varying amounts of a calcium aluminate cement having alow iron content (approx 1%) were employed per 100 parts of the calcinedmaterial so as to give products having a range of nickel oxide contentsbut the same proportion of nickel oxide relative to the diluentmaterials. The products were designated extrudates E, F, and G.

EXAMPLE 6

In this example precipitated basic cobalt carbonate was used in place ofthe precipitated basic nickel carbonate. The product was designatedextrudates H.

The analyses and physical characteristics of extrudates D, E, F, G, andH were as set out in Table 3.

                  TABLE 3                                                         ______________________________________                                                      Extrudates                                                                    D     E      F      G    H                                      ______________________________________                                        Analysis* (%)                                                                 (NiO            19.5    25.9   31.7 36.4 --                                   (CoO            --      --     --   --   27.0                                 (Fe.sub.2 O.sub.3                                                                             5.7     0.5    0.7  0.7  7.5                                  BET surface area (m.sup.2 · g.sup.-1)                                                60.7    26.2   91.3 67.4 39.4                                 Helium density (g · cm.sup.-3)                                                       2.91    2.88   3.05 3.01 3.03                                 Mercury density (g · cm.sup.-3)                                                      2.01    1.85   2.09 2.10 1.87                                 Bulk density (g · cm.sup.-3)                                                         1.18    1.25   1.33 1.25 1.16                                 Porosity (%)    31      36     32   30   38                                   Pores >35 nm (%)                                                                              14      30     22   19   17                                   Pores 15-35 nm (%)                                                                            66      47     49   56   46                                   ______________________________________                                         *After ignition at 900° C.                                        

The extrudates D, E, F, G, and H were tested for hypochloritedecomposition as described in Example 1 in relation to extrudates A. Theresults were as set out in Table 4.

                  TABLE 4                                                         ______________________________________                                        space      NaOCl concentration (g/100 ml)                                     time velocity         outlet                                                  (hr) (hr.sup.-1)                                                                             inlet  D    E     F     G     H                                ______________________________________                                        20   0.87      7.49        0.21                                               20   0.96      7.49              0.01                                         20   1.08      7.49                    0.30                                   22   0.92      7.49   0.74                                                    44   0.74      8.98   0.36                                                    44   0.90      8.98        0.06                                               44   0.96      8.98              <0.01                                        44   1.01      8.98                    0.12                                   93   0.63      8.66                          0.37                             236  0.77      6.68   0.09             0.02                                   236  0.70      6.68        <0.01                                              236  0.75      6.68              <0.01                                        381  0.40      12.34                         0.07                             884  0.90      10.88  0.08 <0.01 <0.01 <0.01                                  ______________________________________                                    

It is seen from these results that with the exception of extrudates F,there is a considerable induction period before the catalyst attains itsfull activity. This is thought to represent the time taken for thecatalyst to become oxidised to the fully active state.

EXAMPLE 7

In this example, the procedure of Example 1 used to make extrudates Awas modified only by employing the pellet mill die giving extrudates,designated herein extrudates I, of diameter about 1.6 mm and lengths inthe range about 3 to 5 mm.

The extrudates I, which contained about 30% nickel oxide, were reducedby heating in a stream of hydrogen at atmospheric pressure and 450° C.and then cooled and stabilised with carbon dioxide and subsequently air.30 ml of the stabilised pre-reduced extrudates were charged to alaboratory reactor provided with a fan effecting internal recycle.Benzene diluted with cyclohexane, and hydrogen diluted with argon, werecontinuously fed to the reactor which was maintained at a pressure of 30bar abs., and at the temperature indicated in Table 5. The products werecontinuously withdrawn and analysed by gas chromatography. The reactionconditions, conversion, and selectivity, were as set out in Table 5.

                  TABLE 5                                                         ______________________________________                                                               feed    fan                                            Temp  C/B*    LHSV.sup.+                                                                             (l/min) speed Conv. Select.                            (°C.)                                                                        ratio   (h.sup.-1)                                                                             H.sub.2                                                                            Ar   (rpm) (%)   (%)                              ______________________________________                                        125   99/1    3.6      1    1.5   140   90.2 >99.9                            150   50/50   3.6      1    0.5  1000  >99.9 >99.9                            180   50/50   3.6      1    0.5  1000  >99.9 >99.9                            ______________________________________                                         *cyclohexane to benzene ratio.                                                .sup.+ ml of liquid (ie benzene plus cyclohexane) fed per hour per ml of      catalyst.                                                                

Similar extrudates J were made having a diameter of about 2 mm andlengths in the range 3 to 5 mm. Also tablets K of diameter 5.4 mm andlength 3.6 mm were made by the procedure used to make tablets B. Whentested on a full scale commercial benzene hydrogenation plant, tablets Khad an activity of about 66% of that of extrudates J. While this may inpart be due to the larger tablet size decreasing the effectiveness ofthe tablets, calculation from the activity of tablets K shows thatsmaller tablets, of equivalent size to extrudates J, would still have anactivity of only about 84% that of extrudates J.

We claim:
 1. In a process for the decomposition of an oxidising agent inan aqueous solution having a pH above 7 wherein said aqueous solution iscontacted with a fixed catalyst bed of shaped particles comprising atleast one oxide of a Group VIII metal M selected from nickel andcobalt,the improvement wherein, in order to provide a catalyst ofincreased activity and/or service life, the catalyst particles comprise10 to 70% by weight, based on the weight of the catalyst particles, ofsaid Group VIII metal oxide (expressed as the divalent oxide, MO), and acalcium aluminate cement, and have a porosity in the range 25 to 50%, inwhich at least 30% of the pore volume is in the form of pores of size inthe range 15 to 35 nm and less than 40% of the pore volume is in theform of pores of diameter greater than 35 nm.
 2. A process according toclaim 1 wherein the pH of the aqueous solution is above
 8. 3. A processaccording to claim 2 wherein the aqueous solution has a pH in the range10 to
 14. 4. A process according to claim 1 wherein the process iseffected at a temperature in the range 20°-80° C.
 5. A process accordingto claim 1 wherein the oxidising agent is hypohalite ions.
 6. A processaccording to claim 5 wherein the aqueous solution is an aqueous effluentsolution wherein the oxidising agent is hypochlorite ions and, aftercontact with said catalyst particles, the aqueous effluent having adecreased concentration of hypochlorite ions is discharged into a river,lake, or estuary.
 7. A process according to claim 1 wherein the catalystparticles also comprise 0.2 to 10% by weight of iron oxide (expressed asFe₂ O₃), based on the weight of the catalyst particles.
 8. A processaccording to claim 1 wherein the catalyst particles comprise calciumaluminate cement in admixture with (i) the Group VIII metal oxide infinely divided form obtained by calcination of a precipitated Group VIIImetal compound and (ii) a finely divided diluent material, the amount ofsaid finely divided diluent material being up to twice the weight ofsaid Group VIII metal oxide.
 9. A process according to claim 8 whereinthe amount of calcium aluminate cement in said particles is 25 to 100%by weight, based on the total weight of said Group VIII metal M oxideand any diluent material.